1. Field of the Invention
The present invention relates to an optical element, to a diffractive optical element and other optical systems having these elements, which are suitable for, for example, an imaging optical system to be used for a camera for forming an image of a photographic subject on a surface of a photosensitive material, an image forming optical system for forming image information on a photosensitive drum by optically scanning the surface of the drum, a projection optical system for projecting electronic circuit patterns on a mask as a first photographic subject on a wafer as a second photographic subject using the projection optical system such as a projection lens for producing a device like a semiconductor element such as an IC and LSI, and an illumination optical system for illuminating the mask for projection as described above.
2. Description of the Related Art
A variety of optical systems using diffractive optical elements employing light diffraction phenomenon have been proposed in recent years. Examples of diffractive optical elements known in the art include Fresnel zone plates, kinoforms, binary optics, and holograms.
Diffractive optical elements are used as optical elements for converting an incident wavefront into a prescribed wavefront. These diffractive optical elements have characteristics that are not found in refractive optical elements. For example, diffractive optical elements have characteristics such as inverse dispersion of the reflective optical elements and the optical system can be compact since the element has substantially no thickness.
Generally speaking, semiconductor manufacturing techniques can be applied for producing diffractive optical elements when it assumes, for example, a binary type configuration, making it possible to relatively easily realize fine pitches. Accordingly, studies on binary type diffractive optical elements in which a blazed configuration is approximated by a stepped structure have been aggressively pursued in recent years.
FIG. 22 to FIG. 24 show illustrative drawings of the main portions of conventional diffractive optical elements.
FIG. 22 shows a Fresnel zone plate, in which a light-shielding member where metallic film is to remain and light-transparent portions where no film is to remain is formed by printing the Fresnel zone by a lithographic process after depositing a metallic film such as a chromium film on a glass substrate. FIG. 23 shows a cross section of a Fresnel lens (kinoform) in which each of the annular periodic patterns along the radius direction follows a continuously curved surface that is formed by cutting or press work. FIG. 24 shows a binary type diffractive optical element comprising a phase difference type diffraction grating machined into steps by repeating plural lithographic processes on the surface of a glass substrate.
FIG. 25 to FIG. 27 show a cross section of the main part of an optical barrel having a conventional diffractive optical element.
In FIG. 25, the diffractive optical element 2501 is inserted into the barrel 2502, the diffractive optical element 2501 having approximately the same effective aperture as that of the barrel 2502. In FIG. 26, the diffractive optical element 2601 is also inserted into the lens barrel 2602 as shown in FIG. 25, the diffractive optical element 2601 having a larger effective aperture than that of the barrel 2602. As shown in FIG. 27, the periphery of the diffractive optical element 2701 is shaved off close to the vicinity of the circumference where the element serves as a diffractive optical element. The reference numeral 2702 denotes the barrel.
Meanwhile, stray light is generated when the light incident on the diffractive optical element impinges on the area outside the diffraction grating, deteriorating optical characteristics.
Accordingly, Japanese Patent Laid-Open Nos. 62-250401 and 4-95233 propose a diffractive optical element in which a light-shielding film is provided outside of the effective area of the diffraction grating.
Various advantages as described above can be obtained when the diffractive optical element is used as a part of the optical system. However, it is difficult, for example, in the diffractive optical element shown in FIG. 25 to assemble it by fitting its effective aperture with the effective aperture of the barrel to leave a portion having no diffraction properties of the diffractive optical element within the effective aperture of the barrel, causing excessive light A to be generated. When the effective aperture of the diffractive optical element is made to be larger than the effective aperture of the barrel as shown in FIG. 26, on the other hand, a problem was encountered in that an excess machining cost was incurred for EB painting of the mask required for machining of the peripheral portion where no light beam should pass through. In addition, fine dust and foreign matter adhere in the diffractive optical element unit, as shown in FIG. 27, since cutting of the portions close to the diffraction grating is required, also causing scattering to occur.
Accordingly, there was a problem in that good quality of diffractive optical elements and optical systems using the optical elements cannot be manufactured because excessive light and scattered light are generated in all the conventional diffractive optical elements.
While stray light is prevented from being generated in the diffractive optical element proposed in Japanese Patent Laid-Open Nos. 62-250401 and 4-95233 cited above by providing a light-shielding film at the periphery of the effective area, detailed constructions of the light-shielding films are not disclosed.
A proper choice of this sort of light-shielding material is crucial, otherwise undesirable substances may be generated from the material by UV irradiation or when gas emitted from the material is decomposed by UV light to generate undesirable substances that fog the lens, thereby shortening the service life of the exposure apparatus. A light-shielding member which is directly exposed to the light is particularly susceptible.